Originally Posted by panhead
Definitely sounds like the antenna cable. The head unit must be grounded.
There's usually a black wire from the new head unit that you can tie into the eyelet of the factory ground strap.
Antenna wires can also be an issue as the trucks age. If there were problems with the factory unit I'd check the ground, and then look into a new antenna cable. Don't even attempt to remove the ole one from behind the dash, just run a new one.